The deal, which has been in the pipeline for five years, will allow both countries to share military intelligence directly without using the US military as an intermediary.
Washington, a close ally of both countries, has long encouraged better relations between the two, even though many in South Korea are critical of the pact.
A spokesman for China’s foreign ministry was critical of the deal, predicting it would increase “insecurity and instability in Northeast Asia.”
South Korea’s Ministry of National Defense in a statement Wednesday highlighted the merits of the deal, saying “Japan’s investment in national defense is higher than South Korea’s which enables the country to monitor and detect military information.”
Seoul is hoping the data will help it to analyze North Korea’s ballistic missile launches and their trajectories as well as the country’s nuclear and submarine capabilities.
In a lengthy article Wednesday, the North Korean state-controlled news agency KCNA accused Washington of going “so far as to breach the basic principles of international laws and ditch its dignity of a superpower by pleading with other countries to join their pressure racket against (North Korea) through downgrading or severing ties with it.”
Japan is the 33rd country with which South Korea has signed a military deal. Under the agreement, which takes place immediately, all confidential and secret information can be shared between the countries’ intelligence agencies. The deal does not cover top secret information.
The intelligence-sharing pact has plenty of critics in South Korea. Japan occupied Korea for 35 years before the end of World War II, and anti-Japanese sentiment still lingers.
Last year, a deal was struck between the two governments over the long-standing issue of Korean sex slaves used by the Japanese military during the war.
Advocacy groups for the so-called comfort women slammed that agreement as a “diplomatic humiliation.”
Outside South Korea, the agreement has other critics.
Chinese foreign ministry spokesman Geng Shuang said at a press conference on Wednesday the intelligence sharing agreement was counterproductive to “the trend of peace and development” in the region.
“Intensified cooperation between relevant countries based on the deeply entrenched Cold War mentality for the sharing of military intelligence will only worsen the confrontation on the Korean peninsula,” he said.
Hundreds of thousands of people have taken to the streets in recent weeks calling for Park’s resignation, while prosecutors named her as a suspect in an ongoing corruption investigation Sunday.
“The Park administration has already lost its legitimacy so she should immediately take her hands off (national affairs),” said Lee Jae-jeong, spokeswoman for the Minjoo opposition party, in a statement.
While emotions are running high, the deal makes sense, according to Daniel Pinkston of Seoul’s Troy University.
“For anyone who ultimately (is) in charge of and responsible for state security, why would she or he decline to receive critical intelligence?” he asked. “It would be irresponsible and a dereliction of duty not to.”
CNN’s James Griffiths contributed to this report.